1. Field of the Invention
The invention relates generally to optical coherence tomography (OCT) and in particular to Fourier domain optical coherence tomography.
2. Description of Related Art
Optical Coherence Tomography (OCT) is a technology for performing high-resolution cross sectional imaging that can provide images of tissue structure on the micron scale in situ and in real time. In recent years, it has been demonstrated that Fourier domain OCT (FD-OCT), which so far employs either a wavelength swept source and a single detector or a broadband source and an array spectrometer, has significant advantages in both speed and signal-to-noise ratio as compared to time domain OCT (TD-OCT). In TD-OCT, the optical path length between the sample and reference arms needs to be mechanically scanned. For example, patent application WO03062802 (EP1470410, CA2474331, US20050018201) is a hybrid time and spectral domain OCT system in which a broadband source is combined with sub-depth range mechanical optical path length scanning and parallel detection of a set of optical spectral bands. This design can increase the signal-to-noise ratio and at the same time reduce the mechanical scanning range. But mechanical scanning is still required and the A-scan speed is thus limited.
In both swept source OCT (SS-OCT) and spectrometer-based spectral domain OCT (SD-OCT), the optical path length difference between the sample and reference arm is not mechanically scanned, instead, a full axial scan (also called A-scan) is obtained in parallel for all points along the sample axial line within a short time determined by the wavelength sweep rate of the swept source (in SS-OCT) or the line scan rate of the line scan camera (in SD-OCT). As a result, the speed for each axial scan can be substantially increased as compared to the mechanical scanning speed of TD-OCT and this is especially beneficial for real-time imaging of movable biological samples such as the human eye. In addition, SD-OCT and SS-OCT can provide substantially greater signal-to-noise ratio relative to TD-OCT, as explained by Mitsui and others (“Dynamic Range of Optical Reflectometry with Spectral Interferometry.” Japanese Journal of Applied Physics 38(10): 6133-6137). There are a number of patents as well as articles that either disclosed the basic concept of or discussed the advantages of Fourier domain OCT using either a swept single wavelength source combined with a single photodetector or a broadband source combined with an array spectrometer. Several of these articles and patents are listed separately under the REFERENCES section. These and other articles and patents cited are all incorporated herein as references of this invention.
However, these prior arts are based on purely employing either a swept single wavelength source combined with a single photodetector (thereafter called pure swept-source OCT or pure SS-OCT) or a broadband source combined with an array spectrometer, comprising an optical spectral dispersing element and an array of photodetectors (thereafter called pure spectral-domain OCT or pure SD-OCT). A pure SS-OCT or a pure SD-OCT system each has its advantages and disadvantages in terms of cost, speed, size, stability and other factors as will be elaborated shortly. Based on the advantages of FD-OCT and most importantly, the cost of currently available optical components, we describe an alternative FD-OCT system that not only retains the advantageous features of a pure SS-OCT and a pure SD-OCT, but also saves the cost of the overall system and increases the speed.
In order to fully appreciate the novel features of the present invention, let us first take a brief look at a pure SS-OCT system and a pure SD-OCT system. FIG. 1 shows the basic configuration of a pure SS-OCT system. Light from a tunable single wavelength laser 102 is split through a beam splitter or fiber coupler 104 into a reference arm 106 and a sample arm 108 of an interferometer and the interference signal is detected with a single high-speed photodetector 110. By sweeping the wavelength of the monochromatic source 102, the spectral interferogram of the OCT interferometer is recorded sequentially. The axial reflectance distribution of the sample is obtained by a Fourier transform of the sequentially acquired detector signal. The most advantageous feature of a pure SS-OCT system, compared to other FD-OCT systems, is its compactness and simplicity. For example, patent application US20050035295 and the article by Oh, W. Y. et al. (“Wide tuning range wavelength-swept laser with two semiconductor optical amplifiers.” Photonics Technology Letters, IEEE 17(3): 678-680) disclosed a wavelength tuning source for SS-OCT that employs a continuously rotating optical arrangement for lasing wavelength selection. In this prior art, a single rotating polygon can be combined with two (or more) gratings and two (or more) optical amplifiers of different optical gain bandwidth to generate a combined wide band wavelength scanning light. The combined output can be synchronized because of the use of a single rotating polygon to provide a continuous linear wavelength tuning over a wide spectral range. However, the current price of a swept source that meets the specification requirement of a practical pure SS-OCT system is very high (see for example, Thorlab Inc. Product Catalog, Vol. 17, (2005) page 469) and in addition, the demonstrated wavelength sweep rate is limited to about 20 kHz (Oh, W. Y. et al. (2005). “Wide tuning range wavelength-swept laser with two semiconductor optical amplifiers.” Photonics Technology Letters, IEEE 17(3): 678-680). Furthermore, commercial products currently having a high price tag are still in the stage of further development, whereas tunable semiconductor lasers developed for optical fiber communications either are step-tuned to fit the ITU grid (see for example, Amano, T. et al. (2005). “Optical frequency-domain reflectometry with a rapid wavelength-scanning superstructure-grating distributed Bragg reflector laser.” Applied Optics 44(5): 808-816) or, if continuously tunable, are very slow (see for example, U.S. Pat. No. 6,847,661) and they do not meet the requirement for a pure SS-OCT system, such as the high wavelength sweeping rate (more than 20 kHz) and the broad spectral range to be covered (e.g. 25 to 200 nm).
FIG. 2 shows the basic configuration of a pure SD-OCT system. Its difference from a pure SS-OCT system is that instead of a wavelength swept laser and a single detector, a broadband source 202 is used and a grating 212 disperses the interfered optical wave to a photodetector array 214. The main disadvantage of a pure SD-OCT system is the bulky size of the spectrometer 216 and the output sensitivity of the spectrometer 216 to mechanical vibration and temperature change. One advantage is the relatively lower cost of the superluminescent diode (˜$1 k) that can be used as the source 202 and a Si (silicon) based line scan camera ($2 k˜$4 k) that can be used as the detector array 214. However, a Si based line scan camera has a limited wavelength response range from 0.25 to 1.1 μm. While this wavelength range is appropriate for some biological imaging applications, longer wavelengths are advantageous in many other applications. For example, melanin pigment and hemoglobin are less absorptive for wavelengths between 1 and 2 μm than for visible light. For OCT in the front portions of the human eye, wavelengths longer than 1 μm offer the advantage of higher illumination power without exceeding eye-safety limits, because water in the eye largely absorbs light of these wavelengths before it reaches the sensitive human retina. As Si cannot cover this wavelength range, InP or InGaAs based detector array appears to be the only practical alternative. Unfortunately, the current price of InP or InGaAs based photodetector array is very high and the line scan rate of these detector arrays is limited to 10 kHz range.
A need therefore exists for an alternative FD-OCT design that can take the speed advantage of FD-OCT without requiring a high-speed array detector, or a high-speed wide-range swept laser source.
The sweep range and sweep speed of a continuously tunable laser are generally limited by the swept filter acting as a tuning element. In addition, laser dynamics limits the sweep speed, especially for longer-cavity lasers. (See for example, Huber, R. et al. (2005). “Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles.” Optics Express 13(9): 3513-3528).
Some examples of sweepable filters that can be used in tunable sources are: 1) piezoelectrically-driven Fabry-Perot filters, 2) unbalanced fiber-based Mach-Zehnder interferometers, 3) distributed Bragg reflectors (DBR), 4) distributed feed-back (DFB) in the gain medium of the laser itself, and 5) rotating gratings outside the laser cavity. For most filter types, especially the first four, smaller sweep ranges are more easily achieved than large sweep ranges. For example, a semiconductor-based DBR, tuned by carrier density, can change the refractive index by 1%, resulting in a tuning range of 1%. The desired tuning range for pure SS-OCT of biological samples is at least 25 nm, which is 2% of a typical laser wavelength of 1310 nm. Each of these filter types can be naturally adapted to pass several wavelengths simultaneously, because they are based on interference and can operate on multiple orders of interference simultaneously.
In this invention, a continuously swept multi-wavelength laser is combined with an optical multi-channel receiver. The multi-wavelength laser emits several optical frequencies or wavelengths simultaneously. This source covers a broad frequency range in a short time by sweeping the set of individual lasing frequencies so that each lasing frequency covers a portion of the full spectrum. The individual lasing optical frequencies are swept over a relatively small range between neighboring lasing frequencies. As a result, the cost of the FD-OCT system can be markedly reduced. The proposed continuously swept multi-wavelength laser can also be made at a low cost as it is only slightly different from standard tunable semiconductor lasers for telecom applications. An additional very beneficial feature of the invention is that the time required for each A-scan can now be substantially reduced, which means that the A-scan rate can be substantially increased relative to the single lasing wavelength swept-source. The individual lasing frequencies simultaneously excite the OCT interferometer, and a multi-channel optical receiver can separate and record the resulting interference signals from the individual optical frequencies. The required multi-channel receiver is now commercially available at a relatively low cost and the price is still continuously dropping as a result of the development for optical fiber telecom applications.